Dual polarized slot elements in septated waveguide cavity

ABSTRACT

A slot radiating element for use in a two-dimensional phased array is in the &#39;&#39;&#39;&#39;end&#39;&#39;&#39;&#39; wall of a septated waveguide which is stepped-down in back of the radiating element to allow for excitation by means of reduced-height waveguide, stripline, coaxial line or other feeding networks without requiring additional vertical or horizontal spacing between the radiating elements. The radiating element constitutes a crossed or vertical slot or circular hole in the end wall centered over the septum with an additional slot in the septum extending into the cavity from the radiating element to excite the horizontal component (component in the plane of the septum) of polarization. Opposite sides of the septum are excited in antiphase (odd mode) to cause excitation of an electric field parallel to the septum in the radiating slot or hole and are fed in-phase (even mode) to cause excitation of an electric field perpendicular to the plane of the septum in the radiating element. Thus, polarization of the radiating slots or hole and, hence, the antenna is controlled by controlling the phase and/or amplitude relationship between the two excitations. In view of the arbitrary height of the slotted end wall, arrays can be stacked close enough to allow the capability of being scanned by phase shift in that plane, without formation of grating lobes.

United States Patent 1 Ajioka l lMarch 13, 1973 i 1 DUAL POLARIZED SLOT ELEMENTS IN SEPTATED WAVEGUIDE CAVITY [75] Inventor: James S. Ajioka, Fullerton, Calif.

[73] Assignee: Hughes Aircraft Company, Culver City,Calif.

22 Filed: Feb. 2, 1972 21 Appl. No.: 222,787

[52] U.S. Cl. .,.,.....343/7 71, 343/840, 343/854 [51] Int. ..II0lq 13/10 [58] Field of Search ..343/771, 840, 854

3,599,216 Paine. ..343/771 Primary Examiner-Eli Lieberman Attorney-W. H. Macallister, Jr. et al.

[57] ABSTRACT A slot radiating element for use in a two-dimensional phased array is in the end" wall of a septated waveguide which is stepped-down in back of the radiating element to allow for excitation by means of reduced-height waveguide, stripline, coaxial line or other feeding networks without requiring additional vertical or horizontal spacing between the radiating elements. The radiating element constitutes a crossed or vertical slot or circular hole in the end wall centered over the septum with an additional slot in the septum extending into the cavity from the radiating element to excite the horizontal component (component in the plane of the septum) of polarization. Opposite sides of the septum are excited in antiphase (odd mode) to cause excitation of an electric field parallel to the septum in the radiating slot or hole and are fed in-phase (even mode) to cause excitation of an electric field perpendicular to the plane of the septum in the radiating element. Thus, polarization of the radiating slots or hole and, hence, the antenna is controlled by controlling the phase and/or amplitude relationship between the two excitations. In view of the arbitrary height of the slotted end wall, arrays can be stacked close enough to allow the capability of being scanned by phase shift in that plane, without formation of grating lobes.

15 Claims, 14 Drawing Figures PATENTEDW13 m 3.720.953

SHEET 10F 4 meaggas ee m I ne 28 P kg 30 L6. E L8. Q $3 $3 23 23 20 A z; a

: 23 $3 23 23 F I g 2 FAlENTEUllllR 13 I975 SHEET 2 BF 4 Parallel plates Feed waveguides Fig. 15.

Parallel plates H6 Parabola PATENTED AR 1975 3, 720,953 SHEET 3 BF 4 C.en1er septum Transverse current 6 +90 90 9 l L 42 X f :1 7 3 I 62 7O Trc|nsverse 62 Transverse slot Slot J Shunt Slots Series Slot Complex Slot ,ATENTE[]MAR13I975 3,720,953

SHEET u 0F 4 Outer conductive layers Fig. 10.

Stripiine 4 Q ii Odd mode G I A l horiz. pol

zzz1r I Lower @z:r- T T l Fig. ll.

DUAL POLARIZED SLOT ELEMENTS IN SEPTATED WAVEGUIDE CAVITY BACKGROUND OF THE INVENTION tled Two-Mode Waveguide Slot Array," by James S.

Ajioka, filed Feb. 9, 1968, and assigned to the same assignee as is the present case. In this patent the slot elements were an integral part of the waveguide feedline, whereas in the present case the radiating elements are completely independent of the feeding lines thereby allowing corporate or multiple beam network or parallel plate feeding techniques.

Other contemporary dual polarized two-dimensional arrays use essentially two separate antenna systems with superposed or interleaved elements for each polarization. In the case of the orthogonally superposed dual edge slot arrays, polarization purity cannot be maintained because of the required inclined slots. Also, the superposed orthogonal array techniques are very narrow band because the antenna beams corresponding to each of the polarizations frequency scan in planes at right angles to each other so that the beams coincide at only one frequency. Otherwise, very narrow band standing wave arrays must be used. Still other systems use dual polarized crossed dipoles which are more complex and more expensive to manufacture than machined slots in the wall of a waveguide cavity.

SUMMARY OF THE INVENTION The present invention is directed towards a radiating element with controllable polarization (i.e., circular, linear, elliptical, dual orthogonal polarization) for use in a two-dimensional phased array that can be fed by waveguide,strip-line, coaxial line or parallel plate (e.g., pillboxes and other geodesic feeds) feeding networks. In general, the phase of the odd mode is controlled by the geometry of the odd mode excitation slot in the septum of the septated waveguide cavity. Since the longitudinal currents in the septum are in phase quadrature with the transverse currents, any phase from 0 to 180 with respect to the feedline power may be achieved by slanting the slot in the septum so that it interrupts both components of current in the desired ratio to effect the desired phase. A bent slot or crossed slot may also be used so that a portion of the slot intercepts the longitudinal current and another portion of it intercepts the transverse current in the desired ratio to effect the desired phase.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a perspective view of a dual polarized element with a crossed slot fed by probes from dual reduced-height waveguide feedlines;

FIG. 2 shows a front view of an array composed of the dual polarized radiating elements of the type shown FIGS. 5 to 9 show different configurations of slots in the center septum;

FIG. 10 shows a cross-sectional view of a waveguide cavity with center septum of the type illustrated in FIGS. 1 and 3 fed with stripline;

FIG. 11 shows a plan view of several waveguide cavities fed with stripline with hybrid for combining the odd and even modes; and

FIGS. 12 and 13 show perspective and cross-sectional views, respectively, of a linear array of dual polarized radiating elements in accordance with the invention, with a parallel plate antenna feed.

DESCRIPTION Referring to FIG. 2, there is illustrated a segment of a slotted array 10 of dual polarized slot elements 12-27 fed by reduced-height waveguides 28-35. In particular, dual polarized slot elements 12-15 are fed by reducedheight waveguides 28, 29; dual polarized slot elements 16-19 are staggered over the elements 12-15 and are fed by reduced-height waveguides 30,31; dual polarized slot elements 20-23 are in alignment with elements 12-15 and are fed by reduced-height waveguides 32,33; and dual polarized slot elements 24-27 are in alignment with elements 16-19 and are fed by reduced-height waveguides 34,35. Embodiments of the dual polarized slot element 12 together with the manner in which the reduced-height waveguides 28,29 are coupled thereto are shown in perspective in FIGS. 1, 3, and 4.

The dual polarized slot elements 12-27 are substantially identical, whereby element 12 is described by way of example. Referring to FIG. 1, a preferred embodiment of dual polarized slot element 12 includes a section of waveguide 40 having an end wall 42 disposed transversely thereacross at the left extremity thereof, as viewed in the drawing, and a symmetrically steppeddown extension 44 at the opposite extremity thereof terminated with an end wall 46 and having steps 47, 48. In the present case, the length of the stepped-down extension 44 is made equal to the width of the reducedheight feed waveguides 28, 29 and steps 47, 48 are each made equal to or greater than the height of the reduced height waveguides 28, 29. Thus, the dual polarized slot elements 12-27 may be stacked with corresponding waveguide portions 40 thereof in actual contact. A septum 50 divides the section of waveguide 40 together with the extension 44 thereto into an upper cavity 52 and a lower cavity 54 of substantially equal volume, as viewed in the drawing.

The reduced-height waveguide 28 is coupled to the upper cavity 52 by means of a conductive probe 55 which extends from the outside broad wall of reducedheight waveguide 28 through an aperture 56 in the common wall between reduced height waveguide 28 and stepped-down extension 44 into the upper cavity 52. Similarly, reduced-height waveguide 29 is coupled to the lower cavity 54 by means of a conductive probe 57 which extends from the outside broad wall of reduced-height waveguide 29 through an aperture 58 in the common wall between reduced-height waveguide 29 and stepped-down extension 44 into the lower cavity 54. The probes 55,57 are generally located opposite each other within the central portion of the stepped-down portions of cavities 52,54.

Next, a radiating clement constituting a crossed-slot 60 is centrally disposed in the end wall 42 with the angle between the legs thereof bisected by the septum 50. Lastly, a slot 62 commencing from the edge of septum 50 coextensive with or within the crossed-slot 60 5 extends into the septum 50 away from the end wall 42. The slot 62 has any one of the configurations shown in FIGS. 5 9. As shown in the drawing, odd mode excitation of the dual feedlines 28,29, i.e., the dual feedlines 28,29 fed in antiphase, produces horizontally polarized excitation of the crossed slot 60. Alternatively, feeding the feedlines 28,29 in phase produces vertically polarized excitation of the crossed slot 60.

Referring again to FIG. 2, the dual polarized slots 12- 15 27 are dispersed along the feedline 28-35 at intervals approximating the guide wavelength, A thereof so as to produce a beam in the vicinity of broadside to the array. Since the guide wavelength, 1 spacing is too great to prevent grating lobes in the horizontal plane as viewed in the drawing, alternate arrays, such as arrays 16-19 and 24-27 are staggered to suppress grating lobe formation in that plane. In the plane normal to feed lines 2835, the spacing is arbitrary and can be made close enough to approximate one half free space wavelength, A /2 at the frequency f,, i.e., at the frequency which produces a beam normal to the array to suppress grating lobes even for wide angle scanning in that plane.

Referring to FIGS. 3 and 3a, there is shown a dual polarized radiating element 70 having a vertical slot 71 in the end wall 42 in place of the crossed slot 60. Other elements of the radiating device of FIG. 3 remain the same as the radiating device of FIG. 1. Referring to FIG. 4, there is shown a dual polarized radiating element 72 having a circular hole 73 in the end wall 42 in place of the crossed slot 60. As before, other elements of the radiator remain the same as the radiating device of FIG. 1. Referring to FIG. 5, there is shown a plan view of slot 62 in the center septum 50. Current which flows parallel to the centerline of center septum 50, i.e., normal to the end wall 42, is designated as longitudinal current and current which flows normal to the longitudinal current is designated transverse current. The phase of the field in the septum slot 62 depends on the angle 0 that the slot 62 makes with the centerline, because 0 determines the relative amounts of longitudinal and transverse currents interrupted by the slot. Inasmuch as the longitudinal and transverse currents are 90 out of phase, the phase of the horizontally polarized component (component parallel to the septurn) can be controlled by the angle 0 of the slot 62 in center septum 50.

FIG. 6 shows a slot array" in the system 50 to excite the horizontally polarized component. The array which is similar to an endfire antenna array allows for precise coupling over a larger bandwidth.

Referring to FIG. 7, there is shown the position of slot 62 with 0= 90 and with 6 =90. Under these circumstances, the relative phase difference between the respective horizontally polarized components is 180. Referring to FIG. 8, the septum slot 62 is shown normal to end wall and displaced from centerline 42, whereby only transverse current is interrupted.

Referring to FIG. 9, there is shown a complex slot 74 in center septum 50 which has a leg portion 75 normal to end wall 42 and a leg portion 76 parallel to end wall 42. The leg portion of slot 74 interrupts transverse currents, and the leg portion 76 interrupts longitudinal currents. The net effect is to generate a field having a phase that is the combination of both the leg portions 75, 76 with the relative lengths of both portions determining the relative phase of the horizontally polarized component of electric field at the radiating element (crossed slot or hole). A slot 62, such as shown in FIG. 5, can produce a field of the same phase by proper selection of the angle 0.

As an alternative to generating progressive phase shifts by propagation along the feed lines 28,29, the dual polarized radiating elements may all be excited with fields of the same phase by means of corporate feeds 80,82 as shown in cross section in FIG. 10. FIG. 10 shows a dual polarized radiating element in accordance with the invention, fed with strip line corporate feeds 80, 82, thereby to energize each of the radiating elements in an array of which the portion 83 of FIG. 11 is shown with fields of the same phase. Referring to FIG. 10, the stripline corporate feeds 80, 82 include outer conductive layers 85, 86 spaced from the center conductors 87 by air or other dielectric material 88. The center conductors 87 as shown in plan view in FIG. 11 form a tree," so that the electrical distance is the same from an input 90 to each output 91, 92, 93, or 94.

In the case of the stripline corporate feeds 80, 82, the center conductors 87 enter the upper and lower cavities 52,54, through the stepped-down portions 48, 47, respectively, and extend parallel to the center septum 50 to probes 90,91. The probes 90,91 are disposed opposite each other normal to the center septum 50 in the central portion of the respective upper and lower cavities 52,54. Tuning screws 96,97 may be threaded through the outer board walls of waveguide section 40 opposite the tips of the probes 90,91. In this case, the crossed slot 60, alternate crossed slot 71, or the circular hole 73 may be used in the end wall 42. The upper and lower stripline corporate feeds 80,82 are either fed in phase or out of phase by means of an appropriate hybrid 98.

Referring to FIGS. 12 and 13, there is shown a perspective and cross-sectional view of a linear array of dual polarized radiating elements with a parallel plate antenna feed 102 of the type referred to as a folded pillbox or other geodesic antenna. The array 100 includes dual polarized radiating elements 103-109, each including the section of waveguide 40 having an end wall 42 and a symmetrically steppeddown extension 44 with steps 47, 48 but no end wall 46. The center septum 50 extends through the combined cavity formed by waveguide section 40, together with extension 44. Although end wall 42 is illustrated with crossed slot 60, the alternate crossed slot 71 or circular hole 73 may also be used. Center septum 50 includes slots of the type illustrated in FIGS. 5 9 and described in connection therewith.

The parallel plate antenna feed 102 includes a cylindrical parabola 112 of a height substantially equal to that of waveguide section 40 and extends across the entire linear array 100 around the back side relative to the radiating elements. The cylindrical parabola 112 is compartmented by conductive plates 113, 114, and

115, plates 113 and 115 being in the plane of the top and bottom surfaces of the waveguide sections 40 of radiators 103-109 and plate 114 being in the plane of the center septum 50. Plate 114 extends from the cylindrical parabola 112 to the center septums 50 of the linear array 100. Plates 113 and 115, on the other hand, extend from the parabola 112 to respective edges which overlap the stepped-down extensions 44 and extend therefrom towards the cylindrical parabola 112 with a gap approximately equal to one-quarter the height of the waveguide sections 40 therebetween. Waveguide inputs 118, 119 along one or both sides of H the steps 47, 48, respectively, to connect with feed horns 120, 121 which are located at the focal point of the cylindrical parabola 112. The input waveguides are fed in the same manner as the corporate feeds in the device of FIG. by the use of appropriate hybrid junctions. Linear arrays 100 may be stacked in the same manner as in the device of FIG. 2.

What is claimed is:

1. A two-dimensional phased array comprising a plurality of rows of contiguous slot radiating elements in a common plane, each of said radiating elements including:

a. uniformly oriented sections of waveguide disposed normal to and commencing from said common plane in a common direction;

. a stepped-down extension in the narrow dimension attached to each of, said uniformly oriented sections of waveguide and extending away from said common plane;

. a conductive end wall disposed across each of said uniformly oriented sections of waveguide in said common plane;

d. a center conductive septum disposed across the broad dimension of each of said uniformly oriented sections of waveguide and said steppeddown extension thereto, thereby to form first and second cavities in each of said sections of waveguide;

. a radiating aperture disposed across said center septum in each of said end walls in said common plane;

f. a slot in each of said center septums commencing from each of said radiating apertures in said common plane, and

. means for exciting said first and second cavities with energy of the type included in the group wherein said first and second cavities are energized in phase and wherein said first and second cavities are energized in anti-phase.

2. The two-dimensional phased array as defined in claim 1 wherein said radiating aperture disposed across said center septum in each of said end walls in said common plane is a longitudinal slot oriented normal to said center septum.

3. The two-dimensional phased array as defined in claim 2 including an additional longitudinal slot in each of said end walls disposed normal to and across said longitudinal slot oriented normal to said center septum.

4. The two-dimensional phased array as defined in claim 1 wherein said radiating aperture disposed across said center septum in each of said end walls in said common plane has a circular configuration.

5. The two-dimensional phased array as defined in claim 1 wherein said radiating aperture disposed across said center septum in each of said end walls in said common plane constitutes first and second crossed longitudinal slots disposed in said end wall with said center septum bisecting the angle therebetween, said crossed longitudinal slots being substantially the same length.

6. The two-dimensional phased array as defined in claim 1 wherein said slot in each of said center septums is at an angle with said end wall, thereby to control the phase of the horizontal polarization component of energy radiated from the concomitant radiating aperture.

7. The two-dimensional phased array as defined in claim 1 wherein said slot in each of said center septums is normal to said end wall and on one side of the centerline of said center septum along said uniformly oriented sections of waveguide, thereby to control the phase of the horizontal polarization component of energy radiated from the concomitant radiating aperture.

8. The two-dimensional phased array as defined in claim 1 wherein said slot in each of said center septums has segments that are normal and parallel to said end wall, thereby to control the phase of the horizontal polarization component of energy radiated from the concomitant radiating aperture.

9. The two-dimensional phased array as defined in claim 8 wherein said slot in each of said center septums has a plurality of spaced segments that are parallel to said end wall.

10. The two-dimensional phased array as defined in claim 8 wherein said segment of said slot in each of said center septums that is normal to said end wall is disposed to one side of the centerline of said center septum along said uniformly oriented sections of waveguide.

11. A two-dimensional phased array comprising a plurality of rows of contiguous slot radiating elements in a common plane, each of said radiating elements including:

a. uniformly oriented sections of waveguide disposed normal to and commencing from said common plane in a common direction;

. a stepped-down extension in the narrow dimension attached to each of said uniformly oriented sections of waveguide and extending away from said common plane;

. conductive end walls disposed across opposite extremities of each of said uniformly oriented sections of waveguide and said stepped-down extensions thereto, thereby to enclose each of said sections of waveguide;

. a center conductive septum disposed across the broad dimension of each of said uniformly oriented sections of waveguide and said steppeddown extensions thereto thereby to form first and second cavities in each of said enclosed sections of waveguide;

. a radiating aperture disposed across said center septum in each of said end walls in said common plane;

f. a slot in each of said center septums commencing from each of said radiating apertures in said common plane; and

g. means for exciting said first and second cavities with energy of the type included in the group wherein said first and second cavities are energized in phase and wherein said first and second cavities are energized in anti-phase.

12. The two-dimensional phased array as defined in claim 11 wherein said last-named means includes first and second strip line corporate feeds coupled to said first and second cavities, respectively, of said radiating elements along a common row.

13. The two-dimensional phased array as defined in claim 11 wherein said last-named means includes first and second lengths of reduced-height waveguide disposed normal to and on opposite sides of said stepped-down extensions of said radiating elements along a common row and coupled to said first and second cavities thereof, respectively.

common plane;

c. a conductive end wall disposed across each of said uniformly oriented sections of waveguide in said common plane,

. a center conductive septum disposed across the broad dimension of each of said uniformly oriented sections of waveguide and said steppeddown extension thereto, thereby to form first and second cavities in each of said sections of waveguide;

e. a radiating aperture disposed across said center septum in each of said end walls in said common plane;

f. a slot in each of said center septums commencing from each of said radiating apertures in said common plane, and

g. means including a parallel plate antenna feed for exciting said first and second cavities of a number of adjacent uniformly oriented sections of waveguide along a row with microwave energy through said respective stepped-down extensions thereto.

15. The two-dimensional phased array as defined in claim 14 wherein said parallel plate antenna feed constitutes an antenna feed of the folded pillbox type.

* is s 

1. A two-dimensional phased array comprising a plurality of rows of contiguous slot radiating elements in a common plane, each of said radiating elements including: a. uniformly oriented sections of waveguide disposed normal to and commencing from said common plane in a common direction; b. a stepped-down extension in the narrow dimension attached to each of said uniformly oriented sections of waveguide and extending away from said common plane; c. a conductive end wall disposed across each of said uniformly oriented sections of waveguide in said common plane; d. a center conductive septum disposed across the broad dimension of each of said uniformly oriented sections of waveguide and said stepped-down extension thereto, thereby to form first and second cavities in each of said sections of waveguide; e. a radiating aperture disposed across said center septum in each of said end walls in said common plane; f. a slot in each of said center septums commencing from each of said radiating apertures in said common plane, and g. means for exciting said first and second cavities with energy of the type included in the group wherein said first and second cavities are energized in phase and wherein said first and second cavities are energized in anti-phase.
 1. A two-dimensional phased array comprising a plurality of rows of contiguous slot radiating elements in a common plane, each of said radiating elements including: a. uniformly oriented sections of waveguide disposed normal to and commencing from said common plane in a common direction; b. a stepped-down extension in the narrow dimension attached to each of said uniformly oriented sections of waveguide and extending away from said common plane; c. a conductive end wall disposed across each of said uniformly oriented sections of waveguide in said common plane; d. a center conductive septum disposed across the broad dimension of each of said uniformly oriented sections of waveguide and said stepped-down extension thereto, thereby to form first and second cavities in each of said sections of waveguide; e. a radiating aperture disposed across said center septum in each of said end walls in said common plane; f. a slot in each of said center septums commencing from each of said radiating apertures in said common plane, and g. means for exciting said first and second cavities with energy of the type included in the group wherein said first and second cavities are energized in phase and wherein said first and second cavities are energized in anti-phase.
 2. The two-dimensional phased array as defined in claim 1 wherein said radiating aperture disposed across said center septum in each of said end walls in said common plane is a longitudinal slot oriented normal to said center septum.
 3. The two-dimensional phased array as defined in claim 2 including an additional longitudinal slot in each of said end walls disposed normal to and across said longitudinal slot oriented normal to said center septum.
 4. The two-dimensional phased array as defined in claim 1 wherein said radiating aperture disposed across said center septum in each of said end walls in said common plane has a circular configuration.
 5. The two-dimensional phased array as defined in claim 1 wherein said radiating aperture disposed across said center septum in each of said end walls in said common plane constitutes first and second crossed longitudinal slots disposed in said end wall with said center septum bisecting the angle therebetween, said crossed longitudinal slots being substantially the same length.
 6. The two-dimensional phased array as defined in claim 1 wherein said slot in each of said center septums is at an angle with said end wall, thereby to control the phase of the horizontal polarization component of energy radiated from the concomitant radiating aperture.
 7. The two-dimensional phased array as defined in claim 1 wherein said slot in each of said center septums is normal to said end wall and on one side of the centerline of said center septum along said uniformly oriented sections of waveguide, thereby to control the phase of the horizontal polarization component of energy radiated from the concomitant radiating aperture.
 8. The two-dimensional phased array as defined in claim 1 wherein said slot in each of said center septums has segments that are normal and parallel to said end wall, thereby to control the phase of the horizontal polarization component of energy radiated from the concomitant radiating aperture.
 9. The two-dimensional phased array as defined in claim 8 wherein said slot in each of said center septums has a plurality of spaced segments that are parallel to said end wall.
 10. The two-dimensional phased array as defined in claim 8 wherein said segment of said slot in each of said center septums that is normal to said end wall is disposed to one side of the centerline of said center septum along said uniformly oriented sections of waveguide.
 11. A two-dimensional phased array comprising a plurality of rows of contiguous slot radiating elements in a common plaNe, each of said radiating elements including: a. uniformly oriented sections of waveguide disposed normal to and commencing from said common plane in a common direction; b. a stepped-down extension in the narrow dimension attached to each of said uniformly oriented sections of waveguide and extending away from said common plane; c. conductive end walls disposed across opposite extremities of each of said uniformly oriented sections of waveguide and said stepped-down extensions thereto, thereby to enclose each of said sections of waveguide; d. a center conductive septum disposed across the broad dimension of each of said uniformly oriented sections of waveguide and said stepped-down extensions thereto thereby to form first and second cavities in each of said enclosed sections of waveguide; e. a radiating aperture disposed across said center septum in each of said end walls in said common plane; f. a slot in each of said center septums commencing from each of said radiating apertures in said common plane; and g. means for exciting said first and second cavities with energy of the type included in the group wherein said first and second cavities are energized in phase and wherein said first and second cavities are energized in anti-phase.
 12. The two-dimensional phased array as defined in claim 11 wherein said last-named means includes first and second strip line corporate feeds coupled to said first and second cavities, respectively, of said radiating elements along a common row.
 13. The two-dimensional phased array as defined in claim 11 wherein said last-named means includes first and second lengths of reduced-height waveguide disposed normal to and on opposite sides of said stepped-down extensions of said radiating elements along a common row and coupled to said first and second cavities thereof, respectively.
 14. A two-dimensional phased array comprising a plurality of rows of contiguous slot radiating elements in a common plane, each of said radiating elements including: a. uniformly oriented sections of waveguide disposed normal to and commencing from said common plane in a common direction; b. a stepped-down extension in the narrow dimension attached to each of said uniformly oriented sections of waveguide and extending away from said common plane; c. a conductive end wall disposed across each of said uniformly oriented sections of waveguide in said common plane, d. a center conductive septum disposed across the broad dimension of each of said uniformly oriented sections of waveguide and said stepped-down extension thereto, thereby to form first and second cavities in each of said sections of waveguide; e. a radiating aperture disposed across said center septum in each of said end walls in said common plane; f. a slot in each of said center septums commencing from each of said radiating apertures in said common plane, and g. means including a parallel plate antenna feed for exciting said first and second cavities of a number of adjacent uniformly oriented sections of waveguide along a row with microwave energy through said respective stepped-down extensions thereto. 